1. Field of the Invention
The present invention relates to a gas separation membrane, a gas separation module, a gas separation device, and a gas separation method.
2. Description of the Related Art
A material formed of a polymer compound has gas permeability specific to the material. Based on this property, it is possible to cause selective permeation and separation out of a target gas component using a membrane formed of a specific polymer compound. As an industrial use aspect for this gas separation membrane related to the problem of global warming, separation and recovery from large-scale carbon dioxide sources with this gas separation membrane has been examined in thermal power plants, cement plants, or ironworks blast furnaces. Further, this membrane separation technique has been attracting attention as a means for solving environmental issues which can be achieved with relatively little energy. In addition, natural gas or biogas (from biological excrement, organic fertilizers, biodegradable substances, sewage, garbage, fermented energy crops, or gas generated due to anaerobic digestion) is mixed gas mainly containing methane and carbon dioxide, and a membrane separation method is being examined as a means for removing impurities such as the carbon dioxide and the like (JP2007-297605A).
When natural gas is purified using the membrane separation method, excellent gas permeability and separation selectivity are required in order to efficiently perform separation of gas. Various membrane materials have been examined in order to realize excellent gas permeability and separation selectivity, and a gas separation membrane using a polyimide compound has been examined as a means therefor. For example, Journal of Membrane Science (2003, 211, pp. 41 to 49) discloses that separation selectivity of a gas separation membrane is improved when a polyimide compound to which a polar group such as a carboxy group or a hydroxyl group is introduced is used.
Moreover, in an actual plant, a membrane is plasticized due to a high pressure condition and impurities (for example, benzene, toluene, and xylene) present in natural gas and this leads to a degradation of separation selectivity, which is problematic. In order to suppress the plasticizing of the membrane, it is known that introduction of a crosslinking structure or a branched structure to a polymer compound constituting the membrane is effective. U.S. Pat. No. 7,247,191B describes a polyimide cross-linked membrane in which a polyimide compound including a carboxyl group is mono-esterified using a diol compound and which allows remaining hydroxyl groups of this diol compound to form a crosslinking structure.
In order to obtain a practical gas separation membrane, it is necessary to ensure sufficient gas permeability by making a gas separation layer thinner and sufficient gas separation selectivity. A method of making a portion contributing to separation into a thin layer referred to as a dense layer or a skin layer by forming a polymer compound such as a polyimide compound into an asymmetric membrane using a phase separation method is known as a technique of forming a gas separation layer into a thin layer. In this asymmetric membrane, a portion other than a dense layer is allowed to function as a support layer responsible for mechanical strength of a membrane.
Further, in addition to the asymmetric membrane, a form of a composite membrane in which a substance responsible for a gas separation function is different from a substance responsible for mechanical strength is also known.